The present invention relates generally to optical communication systems, and in particular, to a method and apparatus that uses an integrated optic component to improve performance and lower costs of free-space optical communication systems.
With the increasing popularity of wide area networks, such as the Internet and/or World Wide Web, network growth and traffic have exploded in recent years. Network users continue to demand faster networks, and as network demands continue to increase, existing network infrastructures and technologies are reaching their limits.
An alternative to existing hardwire or fiber network solutions is the use of wireless optical telecommunications technology. Wireless optical telecommunication systems, also known as xe2x80x9cfree-space opticalxe2x80x9d (FSO) communication systems, utilize beams of light, such as lasers, as optical communications signals, and therefore do not require the routing of cables or fibers between locations. Data or information is encoded into a beam of light, and then transmitted through free space from a transmitter to a receiver. Typically, a pair of transceivers are used to form a communication link, wherein each transceiver provides both receive and transmit functions.
For point-to-point free space laser communications, the use of narrow optical beams provides several advantages, including data security, high customer density, and high directivity. High directivity makes the achievement of high data rates and high link availability easier, due to higher signal levels at a receiver. In order to take full advantage of this directivity, some form of tracking is often necessary to keep the antennas of a transmitter and of the receiver properly pointed at each other. For example, a transmitted optical beam with a 1-mrad divergence has a spot diameter at the receiver of about 1 meter at a 1-km range. Thus, movement of the transmitter or receiver by even a small fraction of the divergence (or field-of-view) could compromise the link unless active tracking is employed. Since high-speed communication channels utilize extremely sensitive detectors, such systems require equally sensitive tracking systems.
Charge coupled device (CCD) arrays or quadrant cell optical detectors (sometimes referred to as xe2x80x9cquad cellsxe2x80x9d) may be used as tracking detectors in a tracking system. In either case, an electrically controllable steering mirror, gimbal, or other steering device may be used to maximize an optical signal (e.g., light) directed at a high-speed detector, based on information provided by the tracking detector. This is possible since optical paths for tracking and communication are pre-aligned, and the nature of a tracking signal for a perfectly aligned system is known. However, at certain wavelengths, a lower wavelength tracking beam is often necessary due to limitations of the detection systems. Such separate wavelengths are typically used with their own set of transmitter optics, thereby requiring the use of additional optical and mechanical hardware. Furthermore, designs using separate beacon and communication optical transmitters require more time in manufacturing because of the need to co-align the two optical transmitters. Such separate transmitter paths are also more susceptible to mis-alignments due to mechanical shock and/or thermal stresses.
A schematic diagram corresponding to a typical optic position correction control loop used in a conventional binocular FSO transceiver is illustrated in FIG. 1. The objective of the control loop is to control the position of a binocular FSO transceiver 10 such that an incoming optical signal having a maximum signal strength: is received by a data detector 12. In the illustrated configuration, an incoming light beam comprising a received optical signal 14 is received by a receive lens 16 and directed toward data detector 12, which is located at the receive lens"" focal point. A beam splitter 18 is disposed in this light path so as to split the received light beam into two portions. The beam splitter directs a majority (e.g., 80-90%) of the beam""s energy toward data detector 12, whereupon the received signal is processed by a signal processing block 20 to generate data 22. A remaining portion (e.g., 10-20%) of the light beam is redirected towards a optical beam position sensor 24. Generally, the optical beam position sensor may comprise a quad cell, CCD, electronic camera, or any other sensor that is capable of detecting the position of a light beam. The beam position sensor generates an error signal (or position or signal strength data from which an error signal can be derived), which is received by a position controller 26. The position controller processes the error signal, position and/or signal strength data to generate a position signal that is used to drive a positioner 28 operatively coupled to the housing of the binocular FSO transceiver to position the transceiver.
In addition to the foregoing signal receiving components, binocular FSO transceiver 10 also includes a set of components used to transmit an outgoing optical signal 30. These components include a data signal generator 32, which drives a transmit laser 34 to produce a optical signal that is collimated (or divergent) by a transmit lens 36 into an optical beam corresponding to outgoing optical signal 30.
This conventional approach has several potential problems. The accuracy of the alignment between the various optical components in the binocular FSO transceiver is critical and must be maintained. One unintended result is that relative physical movement (i.e., drift) between components due to environmental considerations such as vibration and temperature changes may cause changes in the alignments, resulting in performance degradation. For example, suppose that the optical beam position sensor loses alignment with the beam splitter. As a result, when the binocular FSO transceiver and/or its components are positioned so as to produce a maximized signal (based on measurements taken by the optical beam position sensor), the actual signal received by data detector 12 will no longer be a maximum signal. In addition, each optical component must be accurately manufactured and mounted, adding significant cost to the transceiver.
In accordance with one aspect of the invention, an integrated optical component is provided for use in an FSO transceiver. In one embodiment, the integrated optical component comprises a monolithic optically-translucent substrate having a plurality of optics formed therein, including a receiver optic, transmitter optic, a pickoff lens, and a pair of total internal reflection (TIR) fold mirrors. The receiver optic directs a majority of an incoming optical signal towards a data detector, while a small portion of the optical signal passes through the pickoff lens is directed towards a first TIR fold mirror. Upon impinging on a substrate/air interface defined by the first TIR fold mirror, the portion of the optical signal is redirected towards a second TIR fold mirror, which then redirects the portion of light towards a optical beam position sensor. In an optional configuration, only a single TIR fold mirror is used, which directs the portion of the optical signal towards the optical beam position sensor.
In accordance with a second embodiment of the invention, a plurality of tracking lenses and respective TIR fold mirrors are disposed around the periphery of the receiver optic. Respective portions of an incoming optical signal are received by the tracking lenses and redirected by the TIR fold mirrors toward a TIR combiner, which then redirects the respective portions towards a optical beam position sensor.
Additional aspects of the invention concern FSO transceivers in which the various integrated optic component embodiments may be implemented and position control schemes used by the FSO transceivers. In one embodiment, an FSO transceiver is controlled based on position data obtained from its own optical beam position sensor through use of it integrated optic component. In another embodiment, a communication link with a second FSO transceiver is established, and the position of the first FSO transceiver is controlled, at least in part, using position data obtained by a optical beam position sensor corresponding to the second FSO transceiver that is transmitted back to the first FSO transceiver, this is often referred to as xe2x80x9clogin loop pointingxe2x80x9d.